Process for preparation of silver halide emulsion employing a double heat-cycle during finishing

ABSTRACT

The invention is directed to a process for preparation of a silver halide emulsion comprising providing an unsensitized emulsion, heating to carry out chemical sensitization of said emulsion, cooling said emulsion, heating said emulsion a second time to complete heat treatment.

FIELD OF THE INVENTION

The invention relates to the sensitizing of silver halide emulsiongrains for use in a photographic element. The invention particularlyrelates to a process of chemically and spectrally sensitizing silverhalide emulsions, as well as treating emulsions with otherphotographically useful compounds in such a manner as to providesuperior speed/fog performance.

BACKGROUND OF THE INVENTION

In the formation of silver halide emulsions for use in photographicmaterials, a desirable characteristic is to have photographic materialsthat are capable of being rapidly processed. It is known that silverhalide emulsions having high levels of chloride content (greater than 90mole % chloride) are especially useful in achieving rapid processabilitydue to their higher solubility compared to emulsions having greater thansome minimum levels of other halides (for example, emulsions havinggreater than 10 mole % bromide or more than 3-5 mole % iodide or both).

Silver halide emulsions including those high in chloride content arealso known to require some form of chemical sensitization in order toincrease their photographic efficiency. Chemical sensitization of anemulsion involves the addition of one or more chemical sensitizingagents where the sensitizing agent is capable of undergoing a chemicalreaction on the silver halide grain surface during the application ofthermal energy for some time period. The chemical sensitizationinvolving adding chemical sensitizer to an emulsion and heating is oftenreferred to as chemical digestion of the silver halide emulsion.Emulsions, especially high chloride content emulsions, also requirespectral sensitization involving the addition of surface adsorbingsensitizing dyes to the emulsion grains which make the grains sensitiveto specific wavelengths of light. In addition, silver halide emulsiongrains are often treated with other photographically useful chemicalcompounds such as salts of other halides which can cause surfaceconversion of the host emulsion grains to a mixed halide composition.Other known photographically useful compounds which can be added areantifoggants, stabilizers, metal dopants, silver halide solvents,ripeners, supersensitizers, coating aids, and surfactants. Thesephotographically useful compounds can be added prior to, during, orafter the chemical sensitization or the spectral sensitization steps.

It is also known that silver halide emulsions, especially those high inchloride content, can be caused to fog or to exhibit high D_(min)density as a result of the application of heat during the chemicalsensitization (digestion) step. The onset of this undesirable fog orhigh D_(min) is often the limiting factor in the performance ofphotographic materials composed of silver halide emulsions, and there isa continuing need to improve the speed/fog performance of thesesensitized silver halide emulsions, especially emulsions containing highlevels of chloride. The terms "speed" and photographic "sensitivity" areused interchangeably herein.

PROBLEM TO BE SOLVED BY THE INVENTION

There is a continuing need for means to improve the speed/fogperformance of chemically and spectrally sensitized silver halideemulsions.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide a means of obtainingsensitized emulsions which exhibit improved speed performance.

Another object of this invention is to provide a smaller change inemulsion speed with changing temperature of chemical sensitization(digestion) resulting in a more robust performance from emulsionscontaining high levels of chloride.

A further object of this invention is to provide a less heat sensitiveemulsion containing high levels of chloride exhibiting a smaller changein performance with change in temperature of the coated emulsion at thetime of exposure.

These and other objects of the invention are generally accomplished byproviding an unsensitized emulsion, heating to carry out chemicalsensitization of said emulsion, cooling said emulsion, then heating saidemulsion a second time to complete heat treatment.

ADVANTAGEOUS EFFECT OF THE INVENTION

The invention provides silver halide emulsions having improved speedperformance at low fog. The invention provides improved control of thefinishing process and increases the effectiveness of the finishingmaterials used.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph representative of the time and heat cycle forconventional finishing of a silver halide emulsion.

FIG. 2 is a graph of the heat finishing cycle of the invention forsilver halide emulsions.

DETAILED DESCRIPTION OF THE INVENTION

The invention has several advantages over prior processing of silverhalide emulsions. The process of the invention allows independentcontrol of finishing and other treatments. By having separate heatcycles, it allows ideal chemical sensitization followed by moreefficient addition of other materials such as antifoggants, dopants,sensitizing dyes, and material that improved the keeping of theemulsion. It is believed that the chemical sensitizing carried out inthe absence of other finishing materials such as antifoggants,sensitizing dyes, and finish modifiers is often more effective, and thatthe finish modifiers when added after the completion of chemicalsensitization also are more effective in many instances. The separationof chemical sensitization from the heat cycle for other finish materialssuch as antifoggants and sensitizers allows each group of materials tobe added to the silver halide emulsion at the optimum temperature. Theseand other advantages will be apparent from the detailed descriptionbelow.

In FIG. 1 is a time/temperature graph setting forth a conventionalfinishing cycle for a silver halide grain. At lower temperature 12,typically about 40° C., the sensitizing dye 16 and gold and sulfursensitizers 18 are added. The emulsion, which is gelatin and watercontaining silver halide grains, then is ramped in heating from point Kto point I. Point I is typically at about 55° C., and heating takesplace in a typical ramp increase between about 1.5° and 2.0° C. perminute between points K and I. At point I, the temperature is maintainedconstant, while other additives are added to the finish. These typicaladditives are antifoggants 20, supersensitiziers 22, antifoggants 24,dopants 26, and finish modifiers, such as bromide, 28. The temperatureis then maintained constant through J, where the temperature is rampeddown generally at the same rate of change with which it was ramped up.After cooling to the base temperature of about 40° C., there further maybe added pH adjusters 30 and other dyes 32 that are heat sensitive ortoo reactive at high temperatures.

In contrast as shown in FIG. 2, there is a double heat ramp as shown inthe time temperature graph there. The finish shown has the dye 16 andgold and sulfur chemical sensitizers 18 being added prior to the firstheat ramp which goes from point B to point C. The higher temperature of14 is maintained between C and D. Typically the time between C and D isbetween 10 and 20 minutes. Then from point D at temperature 14 thetemperature is ramped down to point E and held for a suitable time suchas 5 minutes, after which the heat ramp begins at F and is ramped up topoint G which is shown as temperature 14 again. Then the other finishingadditives 20, 22, 24, 26, and 28 are added as needed, the elevatedtemperature is maintained until point H after which it is ramped downagain to temperature 12 where material such as pH adjuster 30 and a dye32 may be added.

The temperatures utilized in the first and second elevated heat sectionsof the emulsion treatment may be any suitable temperature that resultsin an improved product. Typically the first heat cycle C-D is to atemperature between about 45° and 80° C. with a preferred elevatedtemperature of between about 55° and 65° C. It is preferred that thesecond temperature B-H during which the finishing material such asantifoggants and supersensitizers are added be between 50° and 60° C. Ittypically, however, may be between about 45° and 70° C. for suitableelements of the invention.

It is preferred that prior to the first heat rise of the invention, agold sulfide and a sensitizing dye be added, and that during the secondelevated temperature cycle, the other finishing materials be added, asit has been found that the most efficient sensitization takes place ifthe sensitizing dye and gold sulfide are placed through a heat cycleprior to the heating with other materials.

The time between heat cycles may be any suitable time. However, forlower cost it is considered desirable that it be kept short, such asabout 5 minutes.

While it has been shown with the preferred embodiment that only chemicalsensitizers and sensitizing dyes be added prior to the first heat cycle,it is also possible that in certain instances other materials, such asdopants, could be added prior to or during the first heat rise. It ispreferred that during the second heat rise, antifoggants,supersensitizing dyes, dopants such as iridium, and additives such asbromide which affect the keeping properties of the emulsion be added.However, it is believed that in certain instances, other combinations ofadditives could be made. Further, it is believed that materials could beadded prior to heating for the second heat rise rather than at theincreased temperature. However, it is considered preferred to addantifoggants, dopants, and keeping additives at the increasedtemperature. It is noted that while the drawing of FIG. 2 illustratesthe first and second heat rise being generally at the same temperature,it is within the invention and indeed preferred that the second heatrise be to a temperature about 5 degrees lower than the first for mosteffective finishing.

Any silver halide grain may be finished in the two-stage finishingmethod of the invention. Typical of such grains are bromoiodide grains,bromide grains, and bromochloroiodide of any morphology including cubicand tabular. Further, tabular silver chloride grains could be utilized.Preferred for the use of the invention are the generally cubical silverchloride grains. As shown in the examples a preferred sensitization hasbeen achieved by the use of the two-stage heat cycle with these cubicalchloride grains.

The silver halide emulsions of this invention can be precipitated by anyof the methods known in the art, for example, those described in T. H.James, The Theory of the Photographic Process (4th Ed.), ResearchDisclosure 36544 of September 1994 in Sections I-III, or ResearchDisclosure 37038 of February 1995 in Section XV. The preferred silverhalide emulsions should be high in chloride content meaning at leastabout 90 mole % chloride, preferably at least about 95 mole % chlorideand most preferably at least 97 mole % chloride. Some bromide mayincorporated during the precipitation, but the most preferred method ofbromide incorporation is addition after the formation of silver chloridegrains by a surface conversion process. The bulk bromide concentrationshould be less than 5 mole %, preferably being no more than 2.5 to 3mole % and most preferably 0.3 to 2.0 mole %. The preferred emulsionshould also contain less than 5 mole % iodide, preferably less that 2mole % and most preferably less than 1 mole % iodide. The source ofeither the added bromide or iodide may be any of the commonly knownsalts, complexes or compounds which can suitably release the halide.

Emulsion precipitation may be conducted in the presence of any of thecommonly known dispersing media including gelatin, synthetic polymers orpeptizers, and conditions of precipitation may include any specificmeans to avoid fog such as control of pAg and pH. Furthermore, otherchemical agents may also be present during the silver halide emulsionprecipitation such as oxidizing agents, antifoggants, sensitizing dyesor other photographically useful compounds as described in ResearchDisclosure 37038 of February 1995 in Section XV. It is specificallycontemplated to use thiosulfonate compounds alone or in combination withsulfinates or selinates during the preparation or treatment of theseemulsions.

It is also contemplated to use one or more of the known useful metaldopants in preparing these high chloride content silver halides in orderto modify the emulsion performance. Metal dopants include salts orcoordination complexes, especially hexacoordination complexes withligands such as halo, aquo, cyano, cyanato, thiocyanoto, nitrosyl, oxoand carbonyl ligands or combinations thereof. Preferred metal dopantsare salts or coordination complexes of Group VIII metals (e.g., iron,ruthenium, rhodium, palladium, osimum, iridium, and platinum). The mostpreferred dopants are salts or complexes of the metals Ir, Os, Ru, andFe. Specific examples of these dopants are described in ResearchDisclosure 37038 of February 1995 in Section XV, Part B.

The sensitization of silver halide emulsions, especially the highchloride content emulsions according to this invention, involvessubjecting the silver halide grains to a double heat treatment whereinthe first heat treatment is the chemical sensitization (digestion) stepand the second heat treatment is a step performed in the presence ofother chemical compounds known to be photographically useful. It iscontemplated that the thermal history of the silver halide emulsionduring the two heat treatment steps will be chosen in such a way thatthe treatment temperature and time will give an optimum final emulsionperformance in terms of speed and fog (D_(min) density). The temperatureand time will depend on the choice and level of the chemical compoundsthat are present in the emulsion before, or during the two heattreatment steps. The temperature of the two heat treatment steps shouldbe greater than that required to simply melt the silver halide emulsionand gelatin mixture, typically above 45° C., but usually less than about80° C. The upper temperature of about 80° C. is determined by the rapidrate of chemical reactions at higher temperature, by thermal degradationof the gelatin or by excessive evaporation, although evaporation may beminimized by covering the reaction vessel during heating.

The chemical sensitization (digestion) can be accomplished by any of avariety of known chemical sensitizing agents such as those described inResearch Disclosure 37038 of February 1995 in Section XV. The preferredsensitizing agents would be sulfiding agents, sources of gold, acombination of sulfur and gold or aqueous colloidal gold sulfide becauseof the ability these agents have to produce substantially higheremulsion speed/fog as compared to the untreated emulsion.

The invention does not rely on the utilization of any particularfinishing materials to achieve the benefit of the double heat-cycle offinishing. Typical of materials suitable for use in the invention,particularly for the preferred color paper, silver chlorides are setforth in Research Disclosure 37038 of February 1995, particularly atpages 888-97.

The following examples illustrate the practice of the invention. Theyare not intended to be exhaustive of all possible variations of theinvention.

EXAMPLE 1

EMULSION A: Precipitation of 10.0 mole Laboratory-scale Silver ChlorideEmulsion

Reactor: Demineralized water, 5.4 liters Gelatin, 38.3 grams/literSodium chloride salt, 2.12 grams/liter Thioether ripener (I), 0.2grams/liter Temperature maintained at 46.0° C.

Solution 1: Silver Nitrate, 475.7 grams/liter

Solution 2: Sodium Chloride, 175.4 grams/liter

Solution 3: Dopant K₄ [Ru(CN)₆ ], 47.3 cc at 3.5 grams/liter

The precipitation is carried out by simultaneously adding solutions 1and 2 to the well-stirred contants of the reactor. Silver nitrate isadded at a rate of 0.465 moles/minute, while the sodium chloride isadded in a manner to maintain the pAg relatively constant in thereactor. The dopant solution (solution 3) is added along with silver andsalt from 75% to 80% of the total mass of added silver. After a totalprecipitation time of 21.5 minutes, the reactor temperature is reducedto 40.0° C. and the silver chloride emulsion grains are desalted bydiafiltration. The final emulsion grain size was determined to beapproximately 0.38 micrometers in mean cubic edge length.

Emulsion Sensitization (Chemical and Spectral) and Treatment with OtherPhotographically Useful Compounds

Desalted emulsion A is divided into smaller samples for treatment asfollows while being well stirred:

SENSITIZATION A: Single Heat Treatment (COMPARISON)

Sample A1: An emulsion sample is melted at 40.0° C., the pH is adjustedto 4.9 and the pAg is adjusted to 7.5 and then,

(a) GSD-1 is added to the emulsion, 25 mg/silver mole.

(b) Colloidal gold sulfide is added to the emulsion, 46.3 mg Au₂ S/mole.

(c) The emulsion is heated to 50.0° C. at 1.67° C./minute.

(d) After 28 minutes at 50.0° C., add AF-1 at 286 mg/mole.

(e) After 40 minutes at 50.0° C., add SS-1 at 60 mg/mole.

(f) After 47 minutes at 50.0° C., add AF-2 at 100 mg/mole.

(g) After 51 minutes at 50.0° C., add MD-1 at 0.047 mg/mole.

(h) After 55 minutes at 50.0° C., add KBr equal to 1.6 mole %.

(i) After 74 minutes at 50.0° C., the emulsion sample is cooled to 40.0°C. at 1.67° C./min.

(j) Sample pH is adjusted to 6.5, add RSD-1, 20 mg/mole.

(k) Emulsion sample is chill-set for later remelting and coating.

Sample A2: This sample is treated like Sample A1 except that the singleheat treatment is at a temperature of 55.0° C.

Sample A3: This sample is treated like Sample A1 except that the singleheat treatment is at a temperature of 60.0° C.

Sample A4: This sample is treated like Sample A1 except that the singleheat treatment is at a temperature of 65.0° C.

Sample A5: This sample is treated like Sample A1 except that the singleheat treatment is at a temperature of 70.0° C.

Sample A6: This sample is treated like Sample A1 except that the singleheat treatment is at a temperature of 75.0° C.

SENSITIZATION B: Double Heat Treatment (INVENTION)

Sample B1: Emulsion sample is melted at 40.0° C., then the pH isadjusted to 4.9 and the pAg is adjusted to 7.5, then,

(a) GSD-1 is added to the emulsion at 25 mg/silver mole.

(b) Colloidal gold sulfide is added to the emulsion at 46.3 mg/mole.

(c) The emulsion is subjected to a first heat treatment by heating to50.0° C. at 1.67° C./minute.

(d) After 17 minutes at 50.0° C., the emulsion sample is cooled to 40.0°C. at 1.67° C./min. and held for 5 minutes.

(e) The emulsion is subjected to a second heat treatment by heating to50.0° C. at 1.67° C./minute.

(f) After 1 minute at 50.0° C., add AF-1 at 286 mg/mole.

(g) After 13 minutes at 50.0° C., add SS-1 at 60 mg/mole.

(h) After 20 minutes at 50.0° C., add AF-2 at 100 mg/mole.

(i) After 24 minutes at 50.0° C., add MD-1 at 0.047 mg/mole.

(j) After 28 minutes at 50.0° C., add KBr equal to 1.6 mole %.

(k) After 47 minutes at 50.0° C., the emulsion sample is cooled to 40.0°C. at 1.67° C./minute.

(l) Sample pH is adjusted to 6.5, add RSD-1 at 20 mg/mole.

(m) Emulsion sample is chill-set for later remelting and coating.

Sample B2: This sample is treated like Sample B1 except that the secondheat treatment is at a temperature of 55.0° C.

Sample B3: This sample is treated like Sample B1 except that the secondheat treatment is at a temperature of 60.0° C.

Sample B4: This sample is treated like Sample B1 except that the secondheat treatment is at a temperature of 65.0° C.

Sample B5: This sample is treated like Sample B1 except that the secondheat treatment is at a temperature of 70.0° C.

Sample B6: This sample is treated like Sample B1 except that the secondheat treatment is at a temperature of 75.0° C.

Sample C1: This sample is treated like Sample B1 except that the firstheat treatment is at a temperature of 55.0° C.

Sample C2: This sample is treated like Sample C1 except that the secondheat treatment is at a temperature of 55.0° C.

Sample C3: This sample is treated like Sample C1 except that the secondheat treatment is at a temperature of 60.0° C.

Sample C4: This sample is treated like Sample C1 except that the secondheat treatment is at a temperature of 65.0° C.

Sample C5: This sample is treated like Sample C1 except that the secondheat treatment is at a temperature of 70.0° C.

Sample C6: This sample is treated like Sample B1 except that the secondheat treatment is at a temperature of 75.0° C.

Sample D1: This sample is treated like Sample B1 except that the firstheat treatment is at a temperature of 60.0° C.

Sample D2: This sample is treated like Sample D1 except that the secondheat treatment is at a temperature of 55.0° C.

Sample D3: This sample is treated like Sample D1 except that the secondheat treatment is at a temperature of 60.0° C.

Sample D4: This sample is treated like Sample D1 except that the secondheat treatment is at a temperature of 65.0° C.

Sample D5: This sample is treated like Sample D1 except that the secondheat treatment is at a temperature of 70.0° C.

Sample D6: This sample is treated like Sample D1 except that the secondheat treatment is at a temperature of 75.0° C.

Sample E1: This sample is treated like Sample B1 except that the firstheat treatment is at a temperature of 65.0° C.

Sample E2: This sample is treated like Sample E1 except that the secondheat treatment is at a temperature of 55.0° C.

Sample E3: This sample is treated like Sample E1 except that the secondheat treatment is at a temperature of 60.0° C.

Sample E4: This sample is treated like Sample E1 except that the secondheat treatment is at a temperature of 65.0° C.

Sample E5: This sample is treated like Sample E1 except that the secondheat treatment is at a temperature of 70.0° C.

Sample E6: This sample is treated like Sample E1 except that the secondheat treatment is at a temperature of 75.0° C.

Sample F1: This sample is treated like Sample B1 except that the firstheat treatment is at a temperature of 70.0° C.

Sample F2: This sample is treated like Sample F1 except that the secondheat treatment is at a temperature of 55.0° C.

Sample F3: This sample is treated like Sample F1 except that the secondheat treatment is at a temperature of 60.0° C.

Sample F4: This sample is treated like Sample F1 except that the secondheat treatment is at a temperature of 65.0° C.

Sample F5: This sample is treated like Sample F1 except that the secondheat treatment is at a temperature of 70.0° C.

Sample F6: This sample is treated like Sample F1 except that the secondheat treatment is at a temperature of 75.0° C.

Sample G1: This sample is treated like Sample B1 except that the firstheat treatment is at a temperature of 75.0° C.

Sample G2: This sample is treated like Sample G1 except that the secondheat treatment is at a temperature of 55.0° C.

Sample G3: This sample is treated like Sample G1 except that the secondheat treatment is at a temperature of 60.0° C.

Sample G4: This sample is treated like Sample G1 except that the secondheat treatment is at a temperature of 65.0° C.

Sample G5: This sample is treated like Sample G1 except that the secondheat treatment is at a temperature of 70.0° C.

Sample G6: This sample is treated like Sample G1 except that the secondheat treatment is at a temperature of 75.0° C.

FORMAT FOR EMULSION PERFORMANCE EVALUATION

A multicolor, multilayer coating was prepared as a photographicrecording element of this invention using the example emulsions in thered sensitive/cyan dye imaging silver halide layer in the followingstructure:

    ______________________________________                                        Gel overcoat layer,   1.076 g gel/sq. meter                                   Red sensitive/cyan dye imaging                                                silver halide layer                                                           Gel at                1.430 g/sq. meter                                       Silver coverage =     0.240 g Ag/sq. meter                                    Coupler CC-1 at       0.422 g/sq. meter                                       Interlayer                                                                    Green sensitive/magenta dye                                                   imaging silver halide layer                                                   Gel at                1.237 g/sq. meter                                       Silver coverage =     0.122 g Ag/sq. meter                                    Coupler MC-1 at       0.355 g/sq. meter                                       Interlayer                                                                    Blue sensitive/yellow dye                                                     imaging silver halide layer                                                   Gel at                1.506 g/sq. meter                                       Silver coverage =     0.218 g Ag/sq. meter                                    Coupler YC-1 at       1.076 g/sq. meter                                       Polyethylene Coated Reflection support                                        ______________________________________                                    

PHOTOGRAPHIC PERFORMANCE EVALUATION

Each of the multicolor, multilayer coatings was exposed by a 1700 Luxtungsten lamp with a 3000° K. temperature for 0.5 seconds followed byprocessing in KODAK EKTACOLOR RA-4 chemistry in a roller transportprocessor. Filtration for the red sensitive layer was a Wratten 70, forthe green sensitive layer a Wratten 99+0.3 neutral density, and for theblue Wratten 48+2B+0.8 neutral density. Emulsion coating performance wasjudged by measuring (a) photographic sensitivity (speed) in relative Logexposure units at a density of 0.8, (b) a lower scale "toe" density at0.2 Log E lower exposure than the speed point, or (c) measuringfog/D_(min) as the lowest density in the unexposed area of the processedcoating.

Table I shows the performance of (comparison) emulsions given a singleheat treatment during which both chemical sensitization and treatmentwith other photographically useful compounds occurred.

                  TABLE I                                                         ______________________________________                                        Emulsion Sensitivity Performance for Single Heat Treatment                               Temperature of Single                                                                        Relative Photographic                               Sample Emulsion                                                                          Heat Treatment Sensitivity (Log exp.)                              ______________________________________                                        A1 (Comparison)                                                                          50° C.  1.52                                                A2 (Comparison)                                                                          55° C.  1.63                                                A3 (Comparison)                                                                          60° C.  1.67                                                A4 (Comparision)                                                                         65° C.  1.67                                                A5 (Comparision)                                                                         70° C.  1.50                                                A6 (Comparision)                                                                         75° C.  1.24                                                ______________________________________                                    

Table II shows the performance of emulsions given a double heattreatment of the invention wherein during chemical sensitization and thetreatment in the presence of other photographically useful compound,temperatures were varied independently.

                                      TABLE II                                    __________________________________________________________________________    Emulsion Sensitivity Performance for Double Heat Treatment                              Second Heat                                                                           Second Heat                                                                           Second Heat                                                                           Second Heat                                                                           Second Heat                                                                           Second Heat                           at 50° C.                                                                      at 55° C.                                                                      at 60° C.                                                                      at 65° C.                                                                      at 70° C.                                                                      at 75° C.                      Sample                                                                            Speed                                                                             Sample                                                                            Speed                                                                             Sample                                                                            Speed                                                                             Sample                                                                            Speed                                                                             Sample                                                                            Speed                                                                             Sample                                                                            Speed                   __________________________________________________________________________    First Heat at 50° C.                                                             B1  1.58                                                                              B2  1.60                                                                              B3  1.60                                                                              B4  1.56                                                                              B5  1.52                                                                              B6  1.26                    First Heat at 55° C.                                                             C1  1.67                                                                              C2  1.66                                                                              C3  1.66                                                                              C4  1.51                                                                              C5  1.52                                                                              C6  1.14                    First Heat at 60° C.                                                             D1  1.75                                                                              D2  1.68                                                                              D3  1.68                                                                              D4  1.63                                                                              D5  1.55                                                                              D6  1.31                    First Heat at 65° C.                                                             E1  1.76                                                                              E2  1.71                                                                              E3  1.68                                                                              E4  1.65                                                                              E5  1.551                                                                             E6  1.22                    First Heat at 70° C.                                                             F1  1.85                                                                              F2  1.72                                                                              F3  1.65                                                                              F4  1.65                                                                              F5  1.54                                                                              F6  1.25                    First Heat at 75° C.                                                             G1  1.74                                                                              G2  1.70                                                                              G3  1.67                                                                              G4  1.66                                                                              G5  1.48                                                                              G6  1.19                    __________________________________________________________________________

It can be seen from Table I and Table II that the invention providessuperior photographic speed than the comparison, especially where thetemperature of the second heating is 50°-60° C. and superiorphotographic speed over a wider temperature range of the first heatingat a second heating temperature of 50°-55° C. At the same time, fog/Dminwas acceptable in all cases.

EXAMPLE 2

An emulsion like that described in EXAMPLE 1 (EMULSION A) is treated asfollows:

Sample H1: (INVENTION) An emulsion sample is melted at 40.0° C., the pHis adjusted to 4.9 and the pAg is adjusted to 7.5 and then,

(a) GSD-1 is added to the emulsion at 20 mg/silver mole.

(b) Colloidal gold sulfide is added to the emulsion at 46.3 mg/mole.

(c) The emulsion is subjected to a first heat treatment by heating to50.0° C. at 1.67° C./minute.

(d) After 17 minutes at 65.0° C., the emulsion sample is cooled to 40.0°C. at 1.67° C./minute and held for 5 minutes.

(e) The emulsion is subjected to a second heat treatment by heating to65.0° C. at 1.67° C./minute.

(f) After 1 minute at 65.0° C., add AF-1 at 286 mg/mole.

(g) After 13 minutes at 65.0° C., add SS-1 at 60 mg/mole.

(h) After 20 minutes at 65.0° C., no AF-2 is added.

(i) After 24 minutes at 65.0° C., add MD-1 at 0.047 mg/mole.

(j) After 28 minutes at 65.0° C., add KBr equal to 1.6 mole %.

(k) After 47 minutes at 65.0° C., the emulsion sample is cooled to 40.0°C. at 1.67° C./min.

(l) Sample pH is adjusted to 6.0, add RSD-1 at 20 mg/mole.

(m) Emulsion sample is chill-set for later remelting and coating.

Sample H2: (INVENTION) Another emulsion sample is treated as sample H1,except that the level of the photographically useful coumpound AF-2added at 65.0° C. during the second heat treatment step is 50 mg/silvermole.

Sample H3: (INVENTION) Another emulsion sample is treated as sample H1,except that the level of the photographically useful coumpound AF-2added at 65.0° C. during the second heat treatment step is 100 mg/silvermole.

Sample H4 and H5: (COMPARISON) Two emulsion samples are treated assample H2 and H3 except that the photographically useful coumpound AF-2is added at 40.0° C. after the second heat treatment (after the"doctoring" step).

Sample H6 and H7: (COMPARISON) Two emulsion samples are treated assample H2 and H3 except that the photographically useful coumpound AF-2is added at 40.0° C. after the second heat treatment (after the"doctoring" step).

These emulsion samples were melted and coated in the same format asdescribed in EXAMPLE 1. Here, however, the emulsion performanceevaluation involved testing their sensitivity to the temperature of thecoating at the time of exposure. Coating samples were exposed at 60° F.and 100° F. to determine the effect of temperature on lower scale toedensity. Table III shows these data.

                                      TABLE III                                   __________________________________________________________________________    Change in Lower Scale Toe Density from 60° F. to 100° F.                   Compound AF-2 added in                                                                       Compound AF-2 added in                                                                       Compound AF-2 added after                       first heating (Comparison)                                                                   second heating (Invention)                                                                   second heating (Comparison)                     Sample Number                                                                         Toe Density                                                                          Sample Number                                                                         Toe Density                                                                          Sample Number                                                                         Toe Density                  __________________________________________________________________________     0.0 mg AF-2/mole                                                                        H1      0.08   H1      0.08   H1      0.08                          50.0 mg AF-2/mole                                                                       H2      0.05   H4      0.04   H6      See Note                     100.0 mg AF-2/mole                                                                       H3      0.03   H5      0.02   H7      See Note                     __________________________________________________________________________

Note: Toe density measurements were not recorded for emulsion samples H6and H7 because very large speed losses were observed with the additionof AF-2 after the second heating which rendered the emulsions too slowto be useful regardless of their low toe properties.

It can be seen from Table III that the invention provides lowersensitivity to coated photographic material temperature during exposurethan the comparison, or retains a higher useful photographic speed wherethe comparison suffers a substantial speed loss. ##STR1##

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. A process for preparation of a silver halide emulsioncomprising providing an unsensitized emulsion, heating to carry outchemical sensitization of said emulsion, cooling said emulsion, heatingsaid emulsion a second time to complete heat treatment wherein saidfirst heating is between 45° and 80° C. and said second heating isbetween 45° and 70° C.
 2. The process of claim 1 wherein chemicalsensitizers utilized during chemical sensitization comprise sulfidingagents, gold sources, combinations thereof or colloidal gold sulfide ingelatin.
 3. The process of claim 1 wherein during said second heatingtime spectral sensitizers and antifoggants are added.
 4. The process ofclaim 3 wherein said cooling is to about 40° C.
 5. The process of claim1 wherein during the second heating antifoggant is added.
 6. The processof claim 1 wherein during the second heating at least one materialselected from dopants, antifoggants, bromide salts or compounds,spectral sensitizers, supersensitizers or dye deaggregants is added. 7.The process of claim 1 wherein chemical sensitizers are added prior toheating to carry out chemical sensitization.
 8. The process of claim 1wherein after said emulsion has been heated a second time, at least onematerial selected from the group consisting of dopants, antifoggants,bromide salts or compounds, spectral sensitizers, supersensitizers, anddye aggregates are added.
 9. The process of claim 1 wherein saidunsensitized emulsion comprises an emulsion that is greater than 90%chloride.
 10. The process of claim 1 wherein said unsensitized emulsioncomprises a silver halide emulsion having a grain size of between about0.15 and about 1.5 micrometers.
 11. The process of claim 10 wherein saidgrain size is between about 0.2 and 1.0 micrometer.
 12. The process ofclaim 1 wherein said first heating is between about 55° and about 65° C.13. The process of claim 1 wherein said heating said emulsion a secondtime is to a temperature of between about 50° and about 60° C.
 14. Theprocess of claim 1 wherein said cooling is to about 40° C.